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Below are pictures of my 1-FET 35-watt Class E transmitter. It uses Steve Cloutier's (WA1QIX) new prototype
Class H modulator board, and a single rail (2 MOSFETs) high voltage / high current Class H modulator output stage (mounted
separately on a heat sink).
It is designed to be very easy to build and operate... and more details will be posted from time-to-time.
With a single RF FET (QFET) the problems associated with stabilizing the behavior of a multi-FET class
E rig are largely avoided.
The main power supply is located on the underside of the chassis. The "wall plug" power consumption of the rig is
about 90 watts. It requires ~5 watts of unmodulated RF drive... and I am using it in conjunction with an FT-1000 MkV transceiver
that easily puts out 5 watts of continuous carrier for this purpose.
As an update: I have recently modified the input circuit to include a 100 ohm, 10 watt swamping resistor in parallel
with the remainder of the input circuit. With the addition of this resistor, I find it easier to maintain a low input SWR.
The drive power remains at ~5 watts. I use a 10:2 toroidal transformer to drive the gate of the single rf FET. The series
input coil (on the 10-turn primary side of the transformer) resonates (approximately) with the input capacitance of the rf
FET... and this results in a 12 volt peak sinousoidal signal on the gate of the rf FET. Note that the 5 watt
drive signal produces a sine wave with approximately 20 volts of peak voltage across the swamping resistor. With the 10:2
transformer, one might expect only 4 volts (20/5) on the gate of the FET. However, the series inductor, resonating with
the FET input capacitance, results in the needed 12 volts of peak voltage on the FET gate.
In the photo below you can see the front side of Steve's (WA1QIX) new prototype class H modulator board. It includes:
the low power audio amplifier and phase inverter stages, the stages that offset the d.c. reference level of
the audio signal to "float" at the proper voltage level for driving the high voltage / high current stage (which
is mounted on the heat sink), the components for the +/- 12 volt (both non-floating and floating) and
+5 volt regulated supplies (except the transformers, which are mounted below the chassis), a circuit for detecting clipping
on negative peaks of modulation, etc.
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From the front, you can see the output tuning coil on the left (20 turns of black, insulated #10 stranded wire
on a 2.25 inch outside diameter PVC pipe), the input tuning coil on the right (27 turns of red, insulated #18 solid wire on
a 1.25 inch outer diameter PVC pipe), the heat sink mounted on the back of the 12 x 8 x 3 inch chassis, and the tuning and
loading capacitors in the middle. You can also see one edge and the back side of the new, prototype WA1QIX class H modulator
board.
In the picture above you can see a closeup view of some of the components that are mounted on the heat sink. Note that
the FETs are attached to the heat sink with #4 screws that fit into holes that have been tapped into the heat sink.
The 3 leads of each FET are connected to terminals on barrier strips. This makes it easy to change out an FET (which,
hopefully, you won't have to do). The source lead of the RF FET (QFET) has an extra wire that is soldered on to it, close
to the body of the device. The other end of this wire is connected to the secondary of the gate drive transformer. This minimizes
the length of the portion of the source circuit path that is shared by the gate current and the drain current. Thus,
the QFET has, in effect, 4 leads... that connect to 4 separate terminals on the corresponding barrier strip: gate, drain,
source, and the auxilliary source lead.
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